add support for unique value renderers based on more than one field

[jgravois]

in general, multi-field unique value renderers just allow the service to define different permutations of unique combinations of the attributes associated with a particular feature in two (or three) fields as opposed to just looking at one. the choice of delimiter used to separate individual values is configurable.

for example a feature like this:

{
  "geometry": {},
  "properties": {
    "name": "somestring",
    "type": "cool"
  }
}

with the default field delimiter , would be appropriately symbolized in a renderer expressed as:

"type": "uniqueValue",
"field1": "name",
"field2": "type",
"field3": null,
"fieldDelimiter": ",",
"uniqueValueInfos": [{
  "symbol": { ... },
  "value": "somestring,cool"
}]

i'm not sure if/where documentation on this exists, but i'm pretty comfortable with the pattern so i'd be more than happy to chat about it in more depth to provide more clarification if it'd be helpful.

[kneemer]

We should do that when time permits.